New Insights into Photochromic Properties of N-Salicylideneaniline Derivatives Using a Cocrystal Engineering Approach

In this contribution, we aim to modify the photochromic properties of anils using a crystal engineering approach. Previous work has shown cocrystallization to alter the photochromic behavior of anils. Here, a full screening of 15 anil derivatives (including 8 newly described anils) with 21 coformers (carboxylic acids, amides, and halogenated benzene derivatives) was performed, resulting in 89 new anil cocrystals. This large amount of results allows drawing statistically valid insights on the complex photochromic mechanism providing evidence of a continuous phenomenon instead of the classic binary photochromic/nonphotochromic one. This is supported by the discovery of “weakly photochromic compounds” presented in this contribution. All the results allowed confirming that an enol-imine character is a requirement and that reducing the amount of short contacts involving the moieties of interest is an interesting way to fine-tune properties as this acts on both electronic and structural aspects. Following this “continuous hypothesis”, the role of the dihedral angle has been explored and shown to be related to the intensity of photochromism instead of being determinant for its occurrence. In addition, in this contribution, we present a novel way of interpreting DRS (diffuse reflectance spectroscopy) data.